1. Field of the Invention
This invention relates to radar systems (and sonar and ladar) and methods for determining the range of objects, and more particularly to radar systems and methods for accurately determining the range of objects having little or no relative velocity.
2. Description of Related Art
Radio Detection and Ranging (xe2x80x9cRadarxe2x80x9d) is commonly employed to detect and determine the range of objects or targets relative to the radar system. FIG. 1 is a diagram of a general radar system 1 and a channel or medium 2 that includes a target 30. As shown in FIG. 1, the radar system includes a transmitter 10 having a transmit antenna 12 and a receiver 20 having a receive antenna 22. In simple terms, the transmitter 10 generates a signal s(t) that is converted to an electromagnetic wave 14 by the transmit antenna 12. The signal travels at the speed of light, c away from the transmit antenna 12 in the medium of the channel 2. The signal may reflect off targets or objects such as the target 30 in the channel 2. The receive antenna 22 receives the reflected electromagnetic waves and generates a signal Sr(t), which is processed by the receiver 20. It is noted that the transmit antenna 12 and the receive antenna 22 may be in close proximity (monostatic radar systems). Alternatively, the transmitter 10 and the receiver 20 may be separated by a large distance (e.g., in bistatic radar systems).
In radar systems, if s(t) is a pulsed signal, the received signal sr(t) is nominally equal to xcex1s(txe2x88x92tr). In such systems, tr is the round trip delay or the time required for the electromagnetic wave to travel from the radar transmit antenna to the target and back to the receive antenna and xcex1 is an amplitude scaling coefficient. In such systems the range of the target is nominally equal to c x tr/2 where c is the speed of light (approximately equal to 3(108)m/s in a vacuum). If the target is moving away from or toward the radar system (i.e., has a non-zero relative velocity), the relative velocity of the target may be determined by calculating the frequency or Doppler shift of s(t). In particular, it is well known that the velocity of the target, v, is nominally equal to xe2x88x92fd*c/f0 where fd is the Doppler frequency and f0 is the frequency of the transmitted wave 14 of s(t). These principles also apply to sonar and ladar (laser-based) target detection and ranging systems. In ladar the velocity of propagation is also the speed of light (the same as for radar). In sonar the velocity of propagation is the speed of sound (which varies with the nature of the medium in the channel). Various radar systems and methods are developed to exploit these well-known attributes to measure the range or velocity of targets in different environments. For example, a prior art system 100 that is used to measure the range and velocity of objects is shown in FIG. 2. As is described below in more detail, the radar system 100 is a homodyned frequency shift keyed (xe2x80x9cFSKxe2x80x9d) diplex radar system. As shown in FIG. 2, the system 100 includes a signal generator or oscillator 101, a transmit antenna 102, a transmit coupler 103, a receive antenna 106, a mixer 104, a switch 108, a dual anti-alias filter 105, and a signal processor 107. The signal generator 101 alternately generates two transmit signals: s1(t)=Cos((xcfx890+xcfx891)txe2x88x92xcex80) and s2(t)=Cos((xcfx890xe2x88x92xcfx891)txe2x88x92xcex80). The signal generator 101 is thus a diplexed signal generator that alternates between the generation of the s1(t) and s2(t) signals. The transmit signals s1(t) and s2(t) are transmitted by the transmit antenna 102 via the transmit coupler 103. The receive antenna 106 receives the reflected signals sr(t) from target objects where the signals are in the form of s(txe2x88x92xcfx84) (switching between s1(txe2x88x92xcfx84) and s2(txe2x88x92xcfx84)). Accordingly, sr(t) is equal to either:
Cos((xcfx890+xcfx891) (txe2x88x92xcfx84)xe2x88x92xcex80)
or
Cos((xcfx890xe2x88x92xcfx891)(txe2x88x92xcfx84)xe2x88x92xcex80).
The received signal sr(t) and the transmit signals s1(t) and s2(t) are downconverted (mixed and low-pass-filtered) by the mixer 104 with the xe2x80x9clocal oscillatorxe2x80x9d (xe2x80x9cLOxe2x80x9d) signal Cos((xcfx890+xcfx891)t) and Cos((xcfx890xe2x88x92xcfx891)t). The variable xcex80 represents the phase delay of the signal between the transmit antenna 102 and the mixer 104 LO signal. The resultant signal is the low pass filter (xe2x80x9cLPFxe2x80x9d) Of sr(t) x s1(t) or s2(t), which is either:
LPF{Cos((xcfx890+xcfx891)t)Cos((xcfx890+xcfx891)(txe2x88x92xcfx84)xe2x88x92xcex80)}=Cos((xcfx890+xcfx891)xcfx84+xcex80)xe2x80x83xe2x80x83Eq. 1
LPF{Cos((xcfx890xe2x88x92xcfx891)t)Cos((xcfx890xe2x88x92xcfx891)(txe2x88x92xcfx84)xe2x88x92xcex80)}=Cos((xcfx890xe2x88x92xcfx891)xcfx84+xcex80).xe2x80x83xe2x80x83Eq. 2
The switch 108 is synchronized to the changes in frequency at the diplexed transmit signal generator 101 and thus generates two different outputs at ports 110 and 112 having signals, F1 and F2 nominally equal to Eq. 1 and Eq. 2 after anti-alias filtering by the dual anti-alias filter 105.
In the above equations, xe2x80x9cxcfx84xe2x80x9d is the round trip propagation delay to the target. By substituting xcfx84=(2/c)(R+Vt) and by letting xcfx89d=xcfx890(2V/c) (note that the Doppler frequency is fd=2Vf0/c), xcex80xe2x80x2=xcfx890(2R/c)+xcex80, xcfx891xe2x80x2=xcfx891(1xe2x88x92(2V/c))≈xcfx891, then xcfx890xcfx84+xcex80=xcfx890(2V/c)t+xcfx890(2R/c)+xcex80=xcfx89dt+xcex80xe2x80x2 and xcfx891xcfx84+xcex81=xcfx891(2V/c)t+xcfx891(2R/c)+xcex81=xcfx891(2Vc)t+xcex81+2xcfx891R/c=xcex81+2xcfx891R/c. Therefore the equations that were written in terms of xcfx84 can also be written as:
F2=Cos(xcfx89dt+xcex80xe2x80x2+2xcfx891R/c))
and
F1=Cos(xcfx89dt+xcex80xe2x80x2xe2x88x922xcfx891R/c)).
Thus, the F1 and F2 signals of the radar system 100 have the same amplitude and frequency but have a different phase. The phase difference between the F1 and F2 signals is xcex94xcfx86=2xcfx891xcfx84=2(2xcfx891R/c)=(4xcfx80(2f1)R/c). Accordingly for this system 100, the range R is computed by the signal processor 107 as follows: R=(xcex94xcfx86)c/(4xcfx80(xcex94f)) where xcex94f=2f1 is commonly called the xe2x80x9cdeviation frequencyxe2x80x9d. Targets of the prior art system (real FSK diplex Doppler radar) appear as signals of the form Cos(xcfx89dt+xcex80xe2x80x2xe2x88x922xcfx891R/c))=Cos(xcfx890(2V/c)t+xcex80xe2x80x2xe2x88x922xcfx891R/c)).
For outbound targets, i.e., targets with increasing range with time, the Doppler shift fd is negative. For inbound targets, i.e., targets with decreasing range with time, the Doppler shift fd is positive. The FFT spectrum for real receivers, however, is always symmetrical about its origin. Specifically, the negative frequency portion of the spectrum is equal to the complex conjugate of the positive frequency portion of the spectrum. It is because of this symmetry that target Doppler signals appearing in any Doppler bin may either be inbound targets or outbound targets, thus there exists a velocity direction ambiguity.
Since the two halves of the spectrum in real receivers contain essentially the same information it is customary in real receivers to only process target information in only one half of the spectrum, e.g., in the positive frequency portion of the spectrum. In the prior art system 101 the direction ambiguity is resolved by observing the polarity of the measured delta phase. Since it is known that target ranges must always be positive it can be inferred whether the target information corresponds to an inbound or outbound target. It must be pointed out that resolving this ambiguity does not resolve inbound and outbound targets in the sense of having independent measurements. It is a weakness of the prior art system that the information for two targets with the same Doppler frequency, e.g., one inbound at +fd and one outbound at xe2x88x92fd, will have their information appearing in the same FFT Doppler bin, resulting in a single corrupted measurement. The resulting measurement cannot be independent for each target since there is only one measurement. If it were possible for the Doppler information for each target to appear in separate FFT Doppler bins then the two targets would actually be resolved in the sense of having independent measurements for each target.
Radars may be utilized in many different applications. In some applications, it may desirable to be able to determine the range of a target that has zero relative velocity. Such a system may be desirable when used in conjunction with a cruise control system in a vehicle or a side-facing radar to detect vehicles in adjacent lanes. Given the equations provided above, it is apparent that the prior radar system 100 is unable to determine the range of a target having zero relative velocity since the phase of the DC Doppler return voltage cannot be measured. In some applications for the radar system 100 this limitation may be undesirable or unacceptable.
In addition to being unable to determine the range of a target having zero relative velocity, the prior art system 100 also has difficulty determining the range of xe2x80x9cfading targetsxe2x80x9d. A target appears as a fading target to a radar system when the radar signal reflected by the target has multiple reflections off the target such as from different points along the surface of a target. The numerous reflections of the signal that are reflected by the target generate constructive and destructive interference. In particular, the reception of multiple signals reflected from a single target can distort the phase of the received signal. In the prior art system 100 shown in FIG. 2, such a distortion of the phase also distorts or limits the accuracy of range determinations.
Finally, the prior art system 100 of FIG. 2 may not be able to resolve range ambiguities. Target range is calculated by a phase measurement. All phase measurements are ambiguous in multiples of 360xc2x0. Therefore, it is possible for the prior art system 100 to detect a target and calculate its range with a large range ambiguity. Consequently, a need exists for a radar system that can accurately determine the range of targets with little ambiguity.
The present invention includes a complex frequency shift keyed homodyned diplexed radar system and method that can accurately determine the range of one or more targets where the targets have little or no velocity relative to the radar system. The system and method generates a FSK electromagnetic wave that is reflected off the one or more targets and converted into a delayed or phase shifted baseband signal and an undelayed baseband signal where the delayed and undelayed baseband signal may be analyzed to determine the range of one or more targets.
In one embodiment, the radar system includes an RF signal generator, a first mixer, a delay circuit and a second mixer. The RF signal generator generates a frequency shifted keyed (FSK) RF signal that is converted to an electromagnetic signal and projected towards one or more targets. The first mixer is coupled to the RF signal generator and mixes a received signal (that is reflected off the one or more targets) and the FSK RF signal to generate a real baseband FSK signal. The delay circuit is coupled to the RF signal generator and delays or phase shifts the FSK RF signal. The second mixer is coupled to the delay circuit and mixes the received signal and the delayed FSK RF signal to generate an imaginary baseband FSK signal. The real baseband FSK signal and the imaginary baseband FSK signal can be used to determine the range of one or more targets where the targets have little or no velocity relative to the radar system.
The radar system may also include a transmit antenna coupled to the RF signal generator where the transmit antenna converts the FSK RF signal to an electromagnetic wave to be directed towards the one or more targets. The radar system may further include a receive antenna coupled to the first mixer and the second mixer. The receive antenna receives electromagnetic waves reflected off the one or more targets and convert the waves to the received signal. The radar system may further include a first switch coupled to the first mixer. The first switch converts the real FSK signal into a first frequency real signal and a second frequency real signal. The system may also include a second switch coupled to the second mixer. The second switch converts the imaginary FSK signal into a first frequency imaginary signal and a second frequency imaginary signal. It is noted that the delay circuit is ideally a phase shifter that phase shifts the FSK RF signal by 90 degrees.
The present invention also includes a method of determining the range of one or more targets having no relative velocity. The method includes generating a frequency shifted keyed (FSK) RF signal and converting the FSK RF signal into an electromagnetic wave directed toward the one or more targets. The method also receives electromagnetic waves reflected from the one or more targets and converts the electromagnetic waves into a received signal. The method mixes the received signal and the FSK RF signal to generate a real baseband FSK signal. The method delays the FSK RF signal and mixes the received signal and the delayed FSK RF signal to generate an imaginary baseband FSK signal. Finally, the method analyzes the real baseband FSK signal and the imaginary baseband FSK signal to determine the range of the one or more targets.
The method may further convert the real FSK signal into a first frequency real signal and a second frequency real signal and convert the imaginary FSK signal into a first frequency imaginary signal and a second frequency imaginary signal. This method then analyzes the first frequency real signal, the second frequency real signal, the first frequency imaginary signal, and the second frequency imaginary signal to determine the range of the one or more targets. As in the system above, the method ideally shifts the phase of the FSK RF signal by 90 degrees.